1. Field of the Invention
The present invention relates to a method of manufacturing an automotive wheel and wheel rim, made out of an aluminum alloy sheet or extrusion.
2. Description of the Related Art
As a method of manufacturing an aluminum alloy wheel for use in a passenger car, truck, buggy, and so forth, there have been known three processes of casting, forging, and press working. An aluminum alloy wheel manufactured by casting is generally heavy in weight (although lighter as compared with a steel wheel), and is expensive, so that it is used mainly for an upscale automobile. An aluminum alloy wheel manufactured by forging is excellent in mechanical property, but very expensive, so that it is used for special purposes. Meanwhile, an aluminum alloy wheel manufactured by press working is advantageous in that it is light in weight, and can be manufactured at a low cost.
In Utility Model Laid-open No. 1988-56935, it is described that a wheel is manufactured by deep drawing a sheet material a plurality of times to thereby form two stepped-cup-like members, and joining the respective bottoms thereof together to be welded along the whole circumference of a joint.
Further, in JP-A No. 4601/1983, there is described a method of forming a wheel (particularly, a wheel rim part thereof), comprising the steps of forming a simple cup-like member by deep drawing a sheet material in two stages, subsequently disposing a metal mold for electromagnetic forming, having a forming face on the external face side thereof, inside the cup-like member, and disposing a coil for electromagnetic forming, on the outer peripheral side of the cup-like member, wherein an instantaneous large current is caused to flow to the coil for electromagnetic forming in the state of a configuration as described as above, and the cup-like member is necked down to be thereby pressed against the forming face of the metal mold for the electromagnetic forming. Incidentally, the electromagnetic forming refers to a technology for forming a workpiece into a predetermined shape taking advantage of a phenomenon where a coil for electromagnetic forming produces a strong magnetic field in a very short time by instantaneously throwing (discharging) electric energy (electric charge) stored at a high voltage into the coil for the electromagnetic forming, and the workpiece placed in the magnetic field is subjected to a strong expansion force or contraction force owing to a repulsive force (Lorentz force according to Fleming's left-hand rule) of the magnetic field, thereby undergoing fast plastic deformation. In the case of this example, the cup-like member is necked down in the direction of the inside diameter by the agency of a strong contraction force to be thereby pressed against the forming face of the metal mold for the electromagnetic forming.
In the case of a method as described in Utility Model Laid-open No. 1988-56935, the two stepped-cup-like members formed by deep drawing the sheet material the plurality of times are required since a drop part at the center of a wheel rim is substantially cylindrical in shape, and is small in diameter while the respective side parts of the wheel rim are larger in diameter, however, the method as described in JP-A No. 4601/1983 has an advantage in that one cup-like member can be formed into the shape of the wheel rim by applying the electromagnetic forming only once. Other methods making use of the electromagnetic forming in the field of the automotive wheel include those disclosed in JP-A No. 79001/1981, and WO No. 00/16927, respectively.
In the case of Utility Model Laid-open No. 1988-56935, and JP-A No. 4601/1983, the wheel comprising a disc integrated with the wheel rim at the outset is manufactured, however, a method of manufacturing a wheel is also conceivable whereby a columnar workpiece is formed into a wheel rim by making use of the electromagnetic forming described in JP-A No. 4601/1983, and subsequently, the wheel rim is combined with a disc separately formed to thereby form the wheel. In this case, if a columnar workpiece fabricated by bending a sheet, and joining both edges of the sheet together by welding, or an extrusion as extruded in a columnar shape can be used as the columnar workpiece, an electromagnetic forming system only is sufficient to form the wheel rim (that is, a large-scale deep-drawing system is unnecessary), so that facilities and a manufacturing process can be significantly simplified.
However, in the case of the method of manufacturing the wheel, comprising the steps of forming the columnar workpiece into the wheel rim, and subsequently, combining the wheel rim with the disc separately formed, joining of the wheel rim with the disc used to pose a problem. Accordingly, the method has been difficult to be made commercially viable. As execution of the joining as described by a simple welding will cause a problem of the formation of thinned material regions, due to weld strain and thermal effect, welding has been difficult to apply. Accordingly, as described in WO No. 00/16927, it has been necessary to execute the joining with the use of a special system/method.
Further, in the case of forming the wheel rim by applying the electromagnetic forming to the columnar workpiece, the following problems have been encountered.                (1) In the case of the electromagnetic forming, the forming of the wheel rim is completed in such a very short time as around several hundred μsec, and the wheel rim is in a peculiar shape (the drop part is small in diameter while respective side parts of the wheel rim, on both sides of the drop part. are larger in diameter), so that there is no time for air existing in gaps between the metal mold and the columnar workpiece to escape at the time of the electromagnetic forming, and the air, in particular, at the drop part in the shape of a concave groove, is trapped between the metal mold and material pressed thereto to be thereby pressurized, causing a problem that the air at high pressure interferes with the material being pressed against a portion of the forming face, at the drop part, resulting in poor forming accuracy (localized bulges are formed on a wheel tread of the wheel rim after the completion of the electromagnetic forming). In order to preclude the problem, orifices or slits, serving as vents, may be provided in the metal mold as appropriate, however, to enable the air to vent sufficiently, there is the need for providing a multitude of the orifices or the slits, largish in size, along the forming face, resulting in an increase in the cost of the metal mold as well as deterioration in design versatility due to the columnar workpiece being pressed against the orifices or the slits to thereby undergo deformation.        (2) In the case of applying the electromagnetic forming to the columnar workpiece so as to neck down the columnar workpiece, the inner peripheral face side of the wheel rim (a face thereof, visible from outside when the wheel rim is attached to an automobile) is butted against the forming face of the metal mold for the electromagnetic forming, and is susceptible to being marred. In the way of making up for such a deficiency, there can be adopted means for face milling throughout the inner peripheral face of the wheel rim to thereby maintain attractive appearance, in which case, however, a problem of an increase in processing cost will arise.        (3) Use of an extrusion for the columnar workpiece is advantageous in that the extrusion has no welded spot, and exhibits substantially uniform material properties throughout the entire circumference thereof. However, the following problem is further encountered in the case of applying the electromagnetic forming so as to neck down a columnar extrusion. That is, because the columnar workpiece needs to have a diameter equal to, or slightly larger than the diameter of a part of the wheel rim after the completion of the electromagnetic forming, largest in diameter, there can be a case where it is difficult to provide a suitable columnar workpiece (the larger the diameter of an extrusion, the more difficult it is to execute extrusion molding) depending on the diameter of the wheel rim, in demand.        